Archive for VIRGO

And then there were five….

Posted in The Universe and Stuff with tags , , , , , , on November 17, 2017 by telescoper

…black hole mergers detected via gravitational waves, that is. Here are the key measurements for Number 5, codename GW170608. More information can be found here.

Here is the abstract of the discovery paper:

On June 8, 2017 at 02:01:16.49 UTC, a gravitational-wave signal from the merger of two stellar-mass black holes was observed by the two Advanced LIGO detectors with a network signal-to-noise ratio of 13. This system is the lightest black hole binary so far observed, with component masses 12+7-2 M⊙ and 7+2-2 M⊙ (90% credible intervals). These lie in the range of measured black hole masses in low-mass X-ray binaries, thus allowing us to compare black holes detected through gravitational waves with electromagnetic observations. The source’s luminosity distance is 340 +140-140Mpc, corresponding to redshift 0.07+0.03-0.03. We verify that the signal waveform is consistent with the predictions of general relativity.

This merger seems to have been accompanied by a lower flux of press releases than previous examples…

Advertisements

GW News Day

Posted in The Universe and Stuff with tags , , on October 16, 2017 by telescoper

Well, it has certainly been an eventful last day in India!

Over a hundred people gathered at IUCAA to see this evening’s press conference, which basically confirmed most of the rumours that had been circulating that a Gamma Ray Burst had been detected in both GW and EM radiation. I won’t write in detail about today’s announcement because (a) a really useful page of resources has been prepared by the group at IUCAA. Check out the fact sheet and (b) I haven’t really had time to digest all the science yet.

I will mention a couple of things, however. One is that the signal-to-noise ratio of this detection is a whopping 32.4, a value that astronomers can usually only dream of! The other is that neutron star coalescence offer the possibility to bypass the traditional `distance ladder’ approaches to get an independent measurement of the Hubble constant. The value obtained is in the range 62 to 107 km s-1 Mpc-1, which is admittedly fairly broad, but is based on only one observation of this type. It is extremely impressive to be straddling the target with the very first salvo.

The LIGO collaboration is over a thousand people. Add to that the staff of no fewer than seventy observatories (including seven in space). With all that’s going in the world, it’s great to see what humans of different nations across the globe can do when they come together and work towards a common goal. Scientific results of this kind will remembered long after the silly ramblings of our politicians and other fools have been forgotten.

I took part in a panel discussion after the results were presented, but sadly I won’t be here to see tomorrow’s papers. I hope people will save cuttings or post weblinks if there are any articles!

UPDATE: Here is a selection of the local press coverage.

Indian LIGO

 

As if these thrilling science results weren’t enough I finally managed to meet my old friend and former collaborator Varun Sahni (who was away last week). An invitation to dinner at his house was not to be resisted on my last night here, which explains why I didn’t write a post immediately after the press conference. Still, of got plenty of papers to read on the plane tomorrow so maybe I’ll do something when I get back.

Tomorrow morning I get up early to return to Mumbai for the flight home, and am not likely to be online again until Wednesday UK time.

Thanks to all at IUCAA (and TIFR) for making my stay so pleasant and interesting. It’s been 23 years since I was last here. I hope it’s not so long before I’m back again!

Gravitational Waves Flash!

Posted in The Universe and Stuff with tags , , , , on October 13, 2017 by telescoper

I got up early this morning to hitch a ride in a car to Mumbai so that I can give a talk this afternoon. We left Pune about 6am and got here about 8.30 so the trip was a quite a bit quicker than coming here! I’ll post about that and include some pictures when I get a moment, but first I’ll post a quick announcement.

There will be an announcement on Monday 16th October at 10am EDT (3pm BST; 7.30pm in Pune) by `the National Science Foundation (NSF) as it brings together scientists from the Laser Interferometer Gravitational-Wave Observatory (LIGO) and Virgo collaborations, as well as representatives for some 70 observatories’. Further details can be found here. The European Southern Observatory has also announced that it will be holding a press conference on Monday about an `unprecedented discovery’.

The fact that it involves LIGO, Virgo and representatives of other observatories strongly suggests that this announcement will address the subject of the rumours that were flying around in August. In other words, it’s likely that on Monday we will hear about the first detecting of a coalescing binary neutron star system with an optical counterpart. Exciting times!

I’ll be back in Pune by Monday and will probably be able to watch the announcement and will update if and when I can.

Gravitational Wave Flash

Posted in The Universe and Stuff with tags , , , on September 27, 2017 by telescoper

Inconveniently timed just before I was due to go to the pub, a new announcement has come out from the LIGO and Virgo gravitational wave detectors. This time it reports a coalescing binary black hole system detected by all three instruments. The new source is called GW170814, which indicates that the signal from it was received by the detectors on the day I returned from Copenhagen this summer!

Here’s the key figure:

The paper is here and there’s a Nature comment piece here.

I have to say that, on its own, the Virgo `detection’ looks rather marginal to me, but assuming that it is a detection this graphic shows how much it helps to localize the source compared to previous signals:

More on this in due course, perhaps, but now I’m off for a pint or two…

LIGO/VIRGO Update

Posted in The Universe and Stuff with tags , , on August 30, 2017 by telescoper

Judging by by the WordPress blog statistics page, there’s been a lot of traffic here in the past week owing to my post about the rumours of a new gravitational wave source detected by LIGO (and possibly VIRGO). In the interest of completeness I’ll just post a quick update to mention that the latest Observation run at LIGO  finished as planned on 25th August, and this has been marked by an official announcement which I have taken the liberty of presenting here in full:

The Virgo and LIGO Scientific Collaborations have been observing since November 30, 2016 in the second Advanced Detector Observing Run ‘O2’ , searching for gravitational-wave signals, first with the two LIGO detectors, then with both LIGO and Virgo instruments operating together since August 1, 2017. Some promising gravitational-wave candidates have been identified in data from both LIGO and Virgo during our preliminary analysis, and we have shared what we currently know with astronomical observing partners. We are working hard to assure that the candidates are valid gravitational-wave events, and it will require time to establish the level of confidence needed to bring any results to the scientific community and the greater public. We will let you know as soon we have information ready to share.

The last two sentences can be translated roughly as “Back off, and give us time to analyse the data!”, which is not an unreasonable request. Judging by the timescale between detection and publication of the previous LIGO events, it will probably be a matter of months before a formal announcement is made.

I hope this clarifies the situation.

 

 

 

LIGO, Leaks and NGC 4993

Posted in Open Access, The Universe and Stuff with tags , , , , on August 23, 2017 by telescoper

No matter what the official policy may be, the more people there are in a collaboration the more likely it is that someone will let their excitement get to their head and start leaking news and starting rumours either directly or indirectly via social media. And so it came to pass last Friday that the following tweet appeared:

I didn’t comment on the time as I thought it might be unreliable – as it indeed it still may be – but now New Scientist has amplified the signal I feel I can’t really be blamed for mentioning it here.

The rumours going round identify the optical counterpart as being in the galaxy NGC 4993 , a red band image of which, from the Second Digitized Sky Survey (DSS2) is shown below:

NGC 4993 is the fuzzy blob slightly above and to the left of the centre of the image. It’s a fairly nondescript lenticular galaxy in a group that can be found in the constellation of Hydra. It lies in the constellation of Hydra, was actually first discovered by William Herschel and it is about 10 arcmin across on the sky. It’s quite nearby, as these things go, with a distance of about 124 million light years (i.e. 40 Mpc or so) and is about 14th magnitude.

If there is an optical counterpart to a gravitational wave event coming from this galaxy then that suggests it may be a coalescence of neutron stars. The black hole mergers that appear to be responsible to the three existing gravitational wave signals that are claimed to have been detected are not expected to release optical light. Confirmation of this interpretation can be found by where the Hubble Space Telescope was pointed yesterday:

Look familiar? HST was, in fact, observing a `BNS-Merger’ (which is short for `Binary Neutron Star’)…

BNS

If this rumour is true then it’s obviously exciting, but there are questions to be asked. Chief among these is how sure is the identification of the counterpart? A transient optical source in NGC4993 may have been observed at the same time as a gravitational wave signal was detected,  but the ability of LIGO to resolve positions on the sky is very poor. On the other hand, the European VIRGO experiment joined Advanced LIGO for the ongoing `O2′ observing run (which ends in a couple of days). Although VIRGO is less sensitive than LIGO having a third detector does improve the localization of the source – assuming, of course, that it detects a signal. Even in that case it certainly won’t be possible to pinpoint the GW source to within 10 arc minutes, which is the precision needed to place it definitely within NGC 4993.

Anyway, we wait and see what, if anything, has been found. If it is a claimed detection then I hope that LIGO and VIRGO will release sufficient data to enable the analysis to be checked and verified. That’s what most of the respondents to my poll seem to hope too!

Much Ado About a Null Result

Posted in Science Politics, The Universe and Stuff with tags , , , on August 20, 2009 by telescoper

In today’s Nature there’s an article outlining the current upper limits on the existence of a stochastic cosmological background of gravitational waves. The basis of the analysis presented in the paper is a combination of data from two larger international collaborations, called VIRGO and LIGO. Cardiff University is a member of the latter, so I suppose I should be careful about what I say…

These experiments have achieved incredible sensitivity – they can measure distortions that are a tiny fraction of an atomic nucleus in scale – but because gravity is such a very weak force they still haven’t managed to find direct evidence of gravitational waves. The next generation of these laser interferometers – Advanced LIGO – should get within hailing distance of a detection but in the meantime we have to do with upper limits. Since the sensitivity of the instruments is so well calibrated, the lack of a signal can yield interesting information. The Nature paper is quite interesting in that it summarizes the constraints that can be placed in such a way on some models of the early Universe. Mostly, though, these are “exotic” models that have already been excluded by other means. If I’ve got my sums right the stochastic gravitational wave background expected to be produced within the standard “concordance” cosmology, in which gravitational wave modes are excited by cosmic inflation, is at least three orders of magnitude lower than current experimental sensitivity.

I can’t resist including the following excerpts from a press release, produced by the Media Relations Department at Caltech whose spin doctors have apparently been hard at work.

Pasadena, Calif.—An investigation by the LIGO (Laser Interferometer Gravitational-Wave Observatory) Scientific Collaboration and the Virgo Collaboration has significantly advanced our understanding the early evolution of the universe.

Analysis of data taken over a two-year period, from 2005 to 2007, has set the most stringent limits yet on the amount of gravitational waves that could have come from the Big Bang in the gravitational wave frequency band where LIGO can observe. In doing so, the gravitational-wave scientists have put new constraints on the details of how the universe looked in its earliest moments.

Much like it produced the cosmic microwave background, the Big Bang is believed to have created a flood of gravitational waves—ripples in the fabric of space and time—that still fill the universe and carry information about the universe as it was immediately after the Big Bang. These waves would be observed as the “stochastic background,” analogous to a superposition of many waves of different sizes and directions on the surface of a pond. The amplitude of this background is directly related to the parameters that govern the behavior of the universe during the first minute after the Big Bang.

and

“Since we have not observed the stochastic background, some of these early-universe models that predict a relatively large stochastic background have been ruled out,” says Vuk Mandic, assistant professor at the University of Minnesota.

“We now know a bit more about parameters that describe the evolution of the universe when it was less than one minute old,” Mandic adds. “We also know that if cosmic strings or superstrings exist, their properties must conform with the measurements we made—that is, their properties, such as string tension, are more constrained than before.”

This is interesting, he says, “because such strings could also be so-called fundamental strings, appearing in string-theory models. So our measurement also offers a way of probing string-theory models, which is very rare today.”

“This result was one of the long-lasting milestones that LIGO was designed to achieve,” Mandic says. Once it goes online in 2014, Advanced LIGO, which will utilize the infrastructure of the LIGO observatories and be 10 times more sensitive than the current instrument, will allow scientists to detect cataclysmic events such as black-hole and neutron-star collisions at 10-times-greater distances.

“Advanced LIGO will go a long way in probing early universe models, cosmic-string models, and other models of the stochastic background. We can think of the current result as a hint of what is to come,” he adds.

“With Advanced LIGO, a major upgrade to our instruments, we will be sensitive to sources of extragalactic gravitational waves in a volume of the universe 1,000 times larger than we can see at the present time. This will mean that our sensitivity to gravitational waves from the Big Bang will be improved by orders of magnitude,” says Jay Marx of the California Institute of Technology, LIGO’s executive director.

“Gravitational waves are the only way to directly probe the universe at the moment of its birth; they’re absolutely unique in that regard. We simply can’t get this information from any other type of astronomy. This is what makes this result in particular, and gravitational-wave astronomy in general, so exciting,” says David Reitze, a professor of physics at the University of Florida and spokesperson for the LIGO Scientific Collaboration.

If hyperbole is what you’re looking for, go no further. There’s nothing wrong with presenting even null results in a positive light but, I don’t think this paints a very balanced picture of the field. For examples, early Universe models involving cosmic strings were already severely constrained before these results, so we know that they don’t have a significant effect on the evolution of cosmic structure anyway.

Clearly the political intention was to flag the importance of Advanced LIGO, although even that will probably be unable to detect the cosmological gravitational-wave background.  Overstatements contained in press releases of this type usually prove counterproductive in the long run.